Many domestic households have two or more telephone extensions installed about the house, but have no facility for intercom between the instruments. The intercom devices that have been available hitherto all offer far more sophisticated a performance than is needed, and are accordingly too expensive for home use.
Telephone systems installed in domestic households are usually provided with four wires looped around the house to the various outlet locations. These wires are conventionally termed and referred to in this specification as the M, N, P and Q wires. Only two of these wires, M and N, are connected to the external telephone wires. Normally, the other two P, Q are redundant.
There have been a number of previous proposals to use the two spare wires in a domestic intercom system.
In Bartelink U.S. Pat. No. 4,485,273 an intercom system is shown which uses a manual switch to change from the external to the intercom wires. This arrangement, however, prevents normal use of the telephone if the switch is left in the intercom position. Bartelink proposes to avoid this by providing a relay to connect the handset to the external wires when the handset of the telephone is replaced but this arrangement has the disadvantage that as soon as the handset is picked up in response to the ringing of an external call, the switch reverts to the intercom position, thereby interrupting the incoming call.
It is an object of the present invention to obviate or mitigate the above disadvantages.